Lasers are widely used as sources of coherent light for many different applications. Compared to incoherent light, coherent laser light can have advantages such as higher intensity, fluence, and brightness levels for a given applied power. Applications where lasers are useful include, but are not limited to, telecommunications, welding, lithography, imaging, material destruction, and holography.
The action by which a laser produces an optical output, or laser action, is a rate process that relies on the excitation of electrons in an optically active medium to upper energy states or levels relative to thermal equilibrium. The addition of excitation energy to the electrons is often referred to as “pumping.” Electrons that have been pumped from a thermal equilibrium state, or ground level, to a higher energy level may be referred to collectively as an “inverted population” or a “population inversion.” In the absence of any triggering process, the excited electrons of an inverted population will spontaneously decay, making one or more transitions to lower energy levels.
An electron making a transition from a high to low energy level can emit a photon, in which case such a transition may be referred to as a “radiative” transition. An electron transition from a high to low energy level may involve heat or momentum transfer, without emitting light, in which case the transition may be referred to as “nonradiative” transition. Often both types of transitions, radiative and nonradiative, occur as an excited electron decays to a lower energy level. Media that undergo radiative transition when excited are sometime referred to as optically “active” or “gain” media, in reference to the optical amplification that can be produced. Laser gain media, as light emitters, have at least one radiative transition available for electrons that have been pumped to sufficiently high energy levels. For optical pumping, i.e., electron excitation via photon absorption, spontaneous radiative transitions to lower energy levels are referred to as fluorescence, when certain conservation of momentum conditions are met.
Excited electrons undergoing radiative transitions spontaneously produce photons, and the resulting emission is referred to as “spontaneous” emission. When an incident photon causes an excited electron to undergo a radiative transition, the resulting emission of light is referred to as “stimulated” emission. The photons produced by stimulated emission have the same direction, wavelength, phase, and polarity, as the incident or triggering photon.
In an optically active medium, or gain medium, the fluorescence or spontaneous radiative decay of an excited electron produces a photon, which can in turn trigger or stimulate other excited electrons to undergo radiative transitions. For a given volume of gain medium, spontaneous emission or fluorescence is randomly produced and distributed over 4π steradians of solid angle, and can stimulate the emission of photons from other excited electrons in the gain medium, thereby amplifying the intensity of the spontaneously emitted photon. This phenomenon is sometimes referred to as amplified spontaneous emission (ASE). Because ASE is random and uniformly occurs over a solid angle of 4π steradians, ASE can deplete or reduce the inverted population that is available for stimulated emission in a desired resonance cavity mode and lead to degradation in performance of an associated laser.
A resonator or cavity is typically used in a laser to select a desired resonance mode, e.g., direction, wavelength, polarization, and phase, from ASE produced in the laser. Since ASE is a nonlinear loss mechanism, ASE that is not amplified by a resonator reduces the efficiency of the associated laser and is problematic with respect to scaling in laser size and pump rate.